This invention relates to temperature control devices and more particularly to a self-calibrating temperature control device that produces substantially precise temperature control while utilizing an ordinarily low precision, large tolerance commerically available temperature sensing element.
The use of temperature sensing elements, such as thermistors, in devices for controlling the temperature of environments of members being heated or being cooled, is well known. Particularly, the use of such devices for controlling the temperature of a heated fuser roller in an electrostatographic copier or printer is known, as disclosed for example in U.S. Pat. No. 4,415,800, issued Nov. 15, 1983 to Dodge et al, and U.S. Pat. No. 4,493,984, issued Jan. 15, 1985 to Yamauchi. Such use, unfortunately, can be unacceptable where it attempts to rely on mass-produced and commerically available temperature sensing elements, such as thermistors, which can have large tolerances ranging from plus or minus 5% to as high as plus or minus 20%. Tolerances of this magnitude, of course, are certain to produce significant temperature control set point errors or variations. In controlling the temperature, for example, of a heated fuser roller in an electrostatographic copier or printer, such errors and variations are undesirable because they can result in poorly fused images, as well as, in curled and even charred copies.
Additionally, the use of such temperature sensing elements can, in the alternative, be very expensive. This is because greater heating or cooling control and precision can be achieved, if small tolerance, high precision elements such as plus or minus 1% tolerance thermistors are used instead of ordinary commercially available ones. However, the cost of 1% thermistors, as an example, is unfortunately much higher than that of malss-produced, commercially available thermistors. In fact, it is not uncommon for the cost of 1% thermistors to be as much as 250% more than the cost of commercially available plus or minus 5% thermistors.